A mutation occurs when a DNA gene is damaged or changed in such a way as to alter the genetic message carried by that gene.
A DNA strand looks like a twisted ladder, and the genes are like a series of letters act like an instruction look and their sequence contains information on building specific molecules, including molecules of protein.
Each cell in our body, depends on thousands of proteins to function in the right places at the right times. Mutation can causes changes in the DNA that result in partially or completely non-functional proteins. It is like a spelling mistake that has occurred. If the affected protein plays a critical role in the body, this can directly or indirectly have many negative effects.